A magnetization direction in a ferromagnetic material has conventionally been utilized as an information bit for a magnetic recording medium. In the nonvolatile magnetic memory (MRAM: Magnetoresistive Random Access Memory), for example, a bit “0” or “1” is determined depending on whether the magnetization directions in a top and bottom layer of a tunnel magnetoresistive element, in which a tunnel barrier layer is sandwiched by the top and bottom layers which are magnetic thin film layers called a free layer and a pinned layer, are parallel or anti-parallel. A word line and a bit line are wired above and below a single magnetic bit (hereinafter called a cell), and the magnetization direction in the free layer can be reversed by making use of a synthetic magnetic field which is generated by applying a current to the word and bit lines.
Since the magnetic field required for the magnetization reversal increases as the ferromagnetic material is miniaturized, a larger current becomes necessary for realizing the magnetization reversal when the size of the cell becomes smaller. Therefore, this technique has a limitation when a large scale integration is taken into consideration.
As a technique to overcome this difficulty, there exists a method to reverse the magnetization by spin injection. This is a technique to reverse the magnetization in the free layer directly by injecting a spin polarized current from the pinned layer. Since the current density required for the magnetization reversal is not significantly influenced by the cell element area, the smaller the cell element, the less current is required for the magnetization reversal. This means that the power consumption needed for writing to a bit could be decreased. The required current density in the state of the art is, however, about 107 A/cm2. Further reduction of the current density is a technical issue toward the low power consumption required for large scale integration.
However, another technical issue will have to be taken into account in the future, that is, when the writing current becomes too low, an output voltage for reading out in turn becomes difficult to obtain, since the current necessary for reading out information must always be lower than the current for writing the information.
The nonvolatile magnetic memory based on the magnetization reversal realized by a magnetic field generated by a current has already been commercially available (see Non-Patent Document 1 below). On the other hand, the memory based on the magnetization reversal realized by spin injection is now at the experimental stage (see Non-Patent Document 2 below).    Non-Patent Document 1: http://www.freescale.com/    Non-Patent Document 2: Digest of technical papers of 2007 IEEE    International Solid-State Circuits Conference (ISSCC 2007), pp. 480-481, February 2007.